Hydrogenation of Formate Species Using Atomic Hydrogen on a Cu(111) Model Catalyst.

The reaction mechanism of the CH3OH synthesis by the hydrogenation of CO2 on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOOa). Thus, the objective of this study is to clarify the reaction mechanism of the CH3OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOOa on Cu(111). We exposed HCOOa on Cu(111) to atomic hydrogen at low temperatures of 200-250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOOa on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H2COOa) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOOa species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOOa species to atomic hydrogen, were detected in the TPD studies. Therefore, H2COOa is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH3OH synthesis via H2COOa from CO2 on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOOa to H2COOa is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO2 on Cu surfaces.

Vibration-driven reaction of CO2 on Cu surfaces via Eley-Rideal-type mechanism.

Understanding gas-surface reaction dynamics, such as the rupture and formation of bonds in vibrationally and translationally excited ('hot') molecules, is important to provide mechanistic insight into heterogeneous catalytic processes. Although it has been established that such excitation can affect the reactions occurring via dissociative mechanisms, for associative mechanisms-in which the gas-phase reactant collides directly with a surface-adsorbed species-only translational excitation has been observed to affect reactivity. Here we report a bond-formation reaction that is driven by the vibrational energy of reactant molecules and occurs via an (associative) Eley-Rideal-type mechanism, in which the reaction takes place in a single collision. Hot CO2 in a molecular beam is found to react with pre-adsorbed hydrogen atoms directly on cold Cu(111) and Cu(100) surfaces to form formate adspecies. The vibrational energy of CO2 is more effective at promoting the reaction than translational energy, the reaction rate is independent of the surface temperature and the experimental results are consistent with density functional theory calculations.